In recent years, there has been progress in miniaturization of a pixel size in a solid-state imaging device such as a CCD (charge coupled device) image sensor or a CMOS (complementary metal oxide semiconductor) image sensor. This leads to a phenomenon in the number of photons that enter a unit pixel, thus leading to lowered sensitivity as well as a lowered S/N ratio. Further, in a case where a color filter is used in which primary color filters of red, green, and blue are two-dimensionally arrayed for colorization, pieces of light of green and blue are absorbed by the color filter, for example, in a red pixel, thus causing the sensitivity to be lowered. Further, an interpolation process is performed between pixels upon generation of each color signal, thus causing occurrence of a so-called false color.
Accordingly, for example, PTL 1 discloses an image sensor using an organic photoelectric conversion film having a multi-layer structure in which an organic photoelectric conversion film having sensitivity to blue light (B), an organic photoelectric conversion film having sensitivity to green light (G), and an organic photoelectric conversion film having sensitivity to red light (R) are sequentially stacked. In this image sensor, the sensitivity is improved by extracting each of the signals B/G/R separately from one pixel. PTL 2 discloses an imaging device in which a single organic photoelectric conversion film is formed, a signal of a single color is extracted with this organic photoelectric conversion film, and signals of two colors are extracted using a silicon (Si) bulk spectroscopy. In so-called laminated imaging devices (image sensors) disclosed in PTL 1 and PTL 2, incident light is mostly subjected to photoelectric conversion and thus read, which allows efficiency of use of visible light to be nearly 100%. Further, color signals of three colors, R, G, and B are obtained at each light-receiving unit, making it possible to generate an image with high sensitivity and high resolution (false color becomes unconspicuous).
For an organic semiconductor serving to absorb green light particularly among organic semiconductors configuring an organic photoelectric conversion film, a subphthalocyanine derivative having superior selectivity in absorption wavelength is widely used. However, the subphthalocyanine derivative is low in carrier mobility, which has caused an issue in which it is not possible to obtain sufficient photoresponse from an imaging device using the subphthalocyanine derivative.
An example of a method of improving conductivity characteristics of a carrier includes a method of doping a target layer with a carrier. For example, PTL 3 discloses a photoelectric conversion element in which transporting a carrier from an anode and a cathode to a photoelectric conversion layer is facilitated by doping with a dopant a photoelectric conversion layer containing poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)], poly[[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophene diyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophene diyl] (PCDTBT).